1. Field of the Invention
This invention pertains generally to plastic article shaping apparatus, and more specifically to preform reshaping using vacuum or suction. The preferred reshaping is accomplished at least in part by creating a reduced pressure on at least one surface of a plastic sheet to provide a differential pressure which is utilized as a shaping force. In one preferred manifestation of the invention, the apparatus is used to form landscape fabric from an indeterminate length web, roll, or spool of plastic film.
2. Description of the Related Art
Modern plastic materials have many desirable features and characteristics that make them advantageously suited for many diverse applications. Among the characteristics is the ability to be formed into diverse shapes. One such shape of interest in the present invention is that of a large sheet of very consistent thickness and nearly indefinite length, referred to herein more simply as a web, sheet or film of indeterminate length. When so formed, the plastic will frequently then be rolled about a spool for storage. Many plastic materials are also very consistent in performance throughout the indeterminate length web, frequently possessing substantial strength to weight ratios.
In addition, many of these same plastics are also heat deformable, which permits them to be warmed to a softened state, and then reshaped into substantially different shapes than the original preform. When the plastic may be heat deformed, cooled to a harder or stiffer state and then heat deformed again, this characteristic is known commonly as being thermoplastic. When a plastic material undergoes a significant chemical change during the heating cycle, typically through a chemical cross-linking or the like, and will not readily undergo further deformation, this is referred to as a thermoset reaction.
Other characteristics that are desirable for certain applications include moisture resistance or impermeability, selective coloring and opacity or transparency, an ability to be laminated with diverse materials either prior to or subsequent to additional processing, mechanical and structural integrity, resistance to many different chemicals and fluids, and, when appropriately treated, resistance to sunlight and ultraviolet radiation.
The ability of plastic materials to be readily reformed permits such materials to be produced with very intricate shapes, without having to expend much time and effort producing each individual component. This economy of labor for individual components is achieved by investing initially in a shaped form referred to as a mold, which is then used to impart the mold shape to many different plastic finished products. There are many different techniques used to mold plastics. For exemplary purposes only, and certainly not representing an exhaustive list, plastics may be injection molded, rotationally molded, blow molded, vacuum thermoformed, and so forth. Regardless of the specific technique, these all provide much benefit in production, since a single mold is used to predictably, quickly and repetitively form many plastic components with very little labor or time invested in the formation of each individual component.
The particular material used for the mold will vary depending upon the particular molding technique and type of plastic material being formed. For injection molding processes with plastics having high viscosity even when fully softened, the mold must sustain enormous deformation forces to adequately reshape the plastic. In such instances, the mold will frequently be of very special steel alloys that are particularly hard and durable. However, when vacuum thermoforming relatively softer and more readily formed materials, such as thin or soft thermoplastic sheets, very little force will ever be applied to the mold, nor will the mold be disposed to erode much. As a result, much softer materials may be used. Aluminum and softer steel alloys are materials that are more commonly used to fabricate a vacuum thermoforming mold.
When processing indeterminate length plastic webs, vacuum thermoforming may be used to shape the plastic sheet in a continuous manner as it is unrolled from the spool. In some prior art processes, the sheet is shaped about individual discrete forms, and is then severed into relatively smaller individual components. Such technique is used, for exemplary purposes, in the formation of plastic plates and bowls. However, other processes maintain the indeterminate length of plastic through the process, by unrolling the plastic from the spool, reshaping the plastic in a continuous manner across a roll, and then wrapping the plastic about a take-up spool or the like.
Exemplary of the prior art processes of continuous vacuum thermoforming is U.S. Pat. No. 3,054,148 by Zimmerli, the teachings which are incorporated herein by reference. Zimmerli discloses an apparatus and process for continuously forming an indeterminate length sheet by passing the sheet over a roller that has vacuum selectively applied to only one segment of the roller. This permits the sheet to be held tightly by the roller in the region of vacuum, and consequently be formed in that region. After being formed, the vacuum is released, permitting the sheet to be readily removed from the forming roll. The forming roll is disclosed as optionally being cooled after the sheet is removed and before again rotating back into contact with the sheet.
Another exemplary patent incorporated herein by reference which illustrates enabling technologies, knowledge and the state of the art is U.S. Pat. No. 4,601,868 by Radel et al, entitled “Method of imparting a three-dimensional fiber-like appearance and tactile impression to a running ribbon of thermoplastic film”. This patent describes a machine that perforates and debosses continuous film by use of a vacuum drum and heated air stream application. The vacuum level is set to three different levels with the rotational position of the film on the drum. U.S. Pat. No. 4,541,794 by Raley et al, entitled “Apparatus for producing perforated plastic film,” and also incorporated by reference, also discloses an embossing and perforating rotating drum system with vacuum varied through the drum position. U.S. Pat. No. 4,741,877 by Mullane, Jr.; U.S. Pat. No. 4,151,240 by Lucas et al; and U.S. Pat. No. 2,809,392 by Armstrong each illustrate additional technologies the teachings of which are incorporated herein by reference.
As will be appreciated, in these prior art forming rolls a hollow cylindrical body is formed by wrapping a perforate sheet or the like into a cylinder. Unfortunately, there is very little flexibility in the geometry of the pattern to be formed, nor, where apertures are desired, in the geometry of and adjacent to the aperture. More recent technologies have attempted to improve upon this limitation, such as by etching or machining three dimensional structures into the perforate sheet. Such production techniques are quite hazardous, expensive, and still only effect microscopic features in the generally two-dimensional sheet. Consequently, this technology has been limited to the forming of primarily flat sheet products, or those products that are dimpled or perforated in repeating pattern corresponding to the holes. In these prior art technologies, the apertures must be normal to the general plane formed by the sheet. Other techniques of vacuum thermoforming using a roll to form a three-dimensional geometry have included wrapping a cloth, fabric or the like upon the roll, about which the sheet good will be formed, thereby creating a three dimensional relief pattern of the cloth in the plastic sheet.
In the field of landscaping, it is known to place indeterminate length plastic sheets upon the ground as a weed barrier. These sheets are commonly imperforate, and consequently block sunlight or plants from passing through. Unfortunately, these imperforate materials prevent moisture and air from passing through also, and so are typically cut in a large perimeter adjacent the plants to permit adequate water and air to reach the plants and soil. Unfortunately, this also permits weeds to grow adjacent the plant. Similar competing technologies use non-woven plastic fabrics as barrier films. These fabrics are designed to allow moisture and air to pass through the film, but prevent larger plants from passing through. Unfortunately, plant roots are sometimes extremely fine and also pass through the non-woven fabrics, enabling a plant to undesirably grow on top of the fabric while degrading the effectiveness of the fabric. These non-woven fabrics are additionally somewhat more expensive and less durable than the imperforate sheets or films of plastic.
An improved landscape fabric has been disclosed in U.S. Pat. No. 5,855,090 by the present inventor, the contents which are incorporated herein by reference. The landscape fabric disclosed therein has controlled perforations formed along vertical walls within the fabric. Drainage channels conveniently direct moisture through the perforations, while sunlight is generally blocked, owing to the generally vertical nature of the perforate walls. However, and as is apparent in the illustrations, the fabric must be formed with relatively complex three-dimensional patterns into an indeterminate length film. While other manufacturing techniques may be used in the production of the fabric disclosed therein, it will be recognized by those skilled in the art of plastics manufacturing that vacuum roll forming is substantially less expensive, where a suitable form may be designed. Unfortunately, the fabrication of a vacuum roll form using prior art techniques such as described by the prior art referenced herein above and incorporated herein by reference requires substantial, almost astronomical expense. This is due to the substantial machining required of the form, whether machined mechanically or chemically. The height differential intrinsic to this improved landscape fabric is substantial enough that a great deal of material must be removed from the form. The prior art technique to achieve this removal is substantial, very repetitive machining. Consequently, the cost of the tooling required for the production of large volumes of the improved fabric has prevented substantial adoption of the fabric within the landscaping industry to date.